How the Works How the Breast Works

In most cases of different sizes and shapes will produce enough breast milk.

Structure of the Breast pressure within the duct, due to expulsion of breast milk from the alveoli and a change in the ’s Alveoli sucking pattern (Ramsay et al., 2004). When breast These are clusters of glandular tissue in the breast milk is no longer being removed, duct size returns where breast milk is produced and stored until it to the resting diameter within 2 minutes due to the is released in response to the infant’s sucking. The backward flow of breast milk(Ramsay et al., 2004). alveoli are secretory acini cells surrounded by a The breast milk ducts at the base of the alveoli are contractile unit of myoepithelial cells (Riordan, 2010). superficial, very small (1.9 ± 0.6 mm), and easily Gaps between the alveolar cells close 2–3 days after compressed. They are easily occluded with gentle , reducing permeability. These alveolar cells pressure (Ramsay et al., 2005). contract to eject the breast milk into the ducts during the letdown or breast milk ejection reflex. There is mammary glandular tissue throughout the breast The mammary papillae form a conical elevation at extending into the axilla; much of the glandular tissue the centre of the . This tissue is very flexible, is located within a 30 mm radius of the base of the able to stretch and mould to conform to the infant’s nipple (Ramsay et al., 2005). mouth. There are usually about 9, ranging from 4 to 18, duct openings or pores where the breast milk Ducts ducts merge and from which breast milk exits the There are tubular structures where breast milk flows breast (Ramsay et al., 2005). The nerve endings are from the alveoli. Ducts and ductules begin branching highly sensitive to stimulation, controlling the release under the areola and intertwine throughout the breast, of breast milk from the ductal openings. surrounded by adipose and glandular tissue, and reaching to the alveoli (Ramsay et al., 2005). Breast milk Areola is ejected into the upper ducts when letdown occurs. The dark area around the nipple comes in a variety of shapes and sizes. It increases in size and colour There are usually about 9 ducts, ranging from 4 to 14, in response to during and opening at the surface of the nipple (Ramsay et al., 2005). . These ducts are for the transport of breast milk, rather Both the nipple and areola are erectile tissue that than for breast milk storage (Geddes, 2007). Previously responds to tactile, sensory and autonomic nervous it was thought that breast milk was stored in widened stimuli (Lawrence, 2010). areas of the ducts, known as milk or lactiferous sinuses, until it was removed through breastfeeding Montgomery’s tubercles contain openings of the or breast milk expression. However, the work of the ducts and sebaceous glands that lubricate the areola. Hartmann et al. (1996), using real-time ultrasound The secretions provide olfactory stimuli for , imagery, has shown that these sinuses do not exist. playing a key role in establishing behavioural and The resting diameter of the duct near the areola is the physiological processes related to breast milk transfer same as the diameter farther up the lobe. and production (Doucet et al., 2009). The ducts increase in diameter when letdown or breast milk ejection reflex occurs during breastfeeding or expression. Dilation of the breast milk duct is caused by a combination of shortening and widening of the duct, together with increased

Breastfeeding Protocols for Health Care Providers | Toronto Public Health 11 Breast Changes During Pregnancy The hormones and are responsible for the increased size and sensitivity of alveoli the breast or during pregnancy. ducts Mammogenesis is the rapid growth and cell development of the alveolar and ductal tissue of the breast that occurs during early pregnancy. nipple These changes are signs that the breasts are preparing { to make breast milk. Beginning from mid-pregnancy to late pregnancy, the breast has the capacity to areola produce the components of breast milk (Lactogenesis I). is normally inhibited during pregnancy because high levels of estrogen and progesterone inhibit the effect of . Lactogenesis II, the initial copious secretion of breast milk 2–4 days after , is triggered by the fall in progesterone and estrogen levels after expulsion of the . This also triggers the closure of the tight junctions between the alveolar cells. At that time, the levels Lactating Breast at Rest of sodium, chloride and protein drop in breast milk, together with an increase in lactose and breast milk lipid levels. Until then, large molecule immunoglobulins and medications pass easily into How Breast Milk is Produced breast milk (Hale et al., 2007). The events of the first The amount of breast milk produced depends on how few days may either promote or delay Lactogenesis often and how effectively the infant removes breast II (Protocol #1: The Initiation of Breastfeeding). milk from the breast. Although the initial copious secretion of breast milk More breast milk will be produced when the infant is driven by hormones rather than by breast milk breastfeeds more frequently and has an effective latch removal, sustained lactation depends on regular and suck. The interaction between storage capacity, Cregan et al., 2002). breast milk removal ( the degree of breast fullness or emptiness, and In most cases, breasts of different shapes and sizes frequency of breast milk removal plays a significant will produce more than enough breast milk. Larger role in breast milk synthesis (Cregan, 1999). breasts have more fatty tissue but not necessarily When the nipple and areola are stimulated by more of the breast milk producing alveoli tissue. infant hand movements and suckling, a cascade Storage capacity, the amount of breast milk stored of hormones are triggered. Hormones are released between breastfeedings, does appear to be related into the mother’s bloodstream, sending messages to to breast size but not to the potential for breast milk the brain via the hypothalamic-pituitary-adrenal or production. Mothers with smaller storage capacities HPA axis. These trigger breast milk production and may need to breastfeed more often and may have less the letdown or breast milk ejection reflex, as well flexibility in the frequency of breastfeeding Hartmann( as other maternal and infant behaviours. Multiple et al., 1996). letdowns occur during a single breastfeeding. Most mothers report sensing the initial letdown but not subsequent ones (Ramsay et al., 2004). Mothers may also not feel letdown but still have a normal letdown response.

BreastfeedingBreastfeeding Protocols Protocols for Health for Health Care Care Providers Providers | Toronto | Toronto Public Public Health Health 12 this as cramps or “after pains”. These cramps may continue for up to 20 minutes after a breastfeeding (Riordan, 2010). is also understood to be a neuropeptide that plays a role in influencing emotional and physiological responses. These include cardiovascular, digestion, appetite and metabolism, pain threshold, kidney function, thermoregulation and stress response behaviours (Uvnus-Moberg, 1998), as well as attachment behaviours. These early maternal attachment behaviours promote infant brain development and plasticity (Schore, 2001). A feeling of calm often reported by women during breastfeeding is partly mediated by oxytocin (Riordan, 2010). It has Lactating Breast after Letdown been termed an agent of the calm and connection response (Uvnus-Moberg et al., 2005). Prolactin and oxytocin play a role in maternal Prolactin is the synthesized, stored and adaptation to stress, whereby the responses of the released from the to stimulate HPA axis and the sympathetic nervous system to the alveoli to produce and secrete breast milk. The emotional and physical stressors are attenuated or greater the stimulation to the nipple and areola, blunted (Slattery et al., 2008), facilitating right-brain and the more prolactin that is secreted by the anterior intuitive behaviours. , leading to more breast milk being Other hormones also have significant roles in produced. Prolactin levels have diurnal variations lactation. These include but are not limited to: following a circadian rhythm, with higher levels occurring at night (Riordan, 2010; Hale et al., 2007). • Glucosteroids, which contribute to alveolar Levels of prolactin are highest in early lactation development, the regulation of fat, protein and and decline as lactation progresses (Cox et al., 1996, lactose synthesis, and water transport across cell Hale et al., 2007). Prolactin levels may be regulated membranes. by a feedback mechanism known as the Prolactin • , important to stress mediation and normally – Inhibiting Factor (PIF) or Feedback Inhibitor of lactogenic; the high levels produced during stress Lactation (FIL), a dopamine-related substance that may be antagonistic to lactation. inhibits the uptake of prolactin by the prolactin • Thyroid-stimulating hormone, which facilitates receptors (Lawrence, 2010). Frequent breastfeeding in the early days may stimulate development of mammary growth and lactation. prolactin receptors, resulting in greater breast milk Breast milk is a dynamic, living substance (Walker, output (De Carvalho et al., 1983). 2010). It has many, many bioactive components, and Oxytocin is the hormone that sends messages to the transitions continually to meet the changing needs of alveoli to eject breast milk into the ducts. In response the infant. These transitions occur on a continuum, with the composition of breast milk varying to suckling and infant hand massage (Matthiesen et al., according to the stage of lactation, the time of day, 2001), the posterior pituitary gland secretes oxytocin, causing breast milk ejection reflex or letdown to the sampling time during a given feeding, maternal contract the myoepithelial cells of the alveoli and nutrition and individual variation (Lawrence, 2010). eject breast milk into the ducts. The breast milk Traditional nomenclature suggests distinct is drawn down the dilated ducts toward the areola differences between “colostrum”, “transitional milk” to be removed during breastfeeding or breast milk or “mature milk”, and between “foremilk” and expression. Oxytocin dilates the ducts during “hindmilk”, which can imply limited value and use breast milk ejection reflex and also sends messages (Lawrence, 2010). Some cultures discard the first milk. to the uterus to contract. This helps to prevent However, it is important to acknowledge that all maternal haemorrhaging and to promote uterine breast milk is “milk” and is valuable for infants. involution after birth. Mothers may experience The first breast milk (sometimes known as

Breastfeeding Protocols for Health Care Providers | Toronto Public Health 13 colostrum) is produced during late pregnancy and Breast milk also changes throughout each in the early , and provides the breastfeeding, particularly related to fat content. infant with nutrition and protection against infectious At the initiation of a breastfeeding, the breast disease (Lawrence, 2010). Typically, it is a clear milk has a small amount of fat, but the fat content yellow, viscous fluid rich in proteins (reflecting the rises throughout the breastfeeding in relation to presence of a large number of immunoglobulins), the degree of breast emptiness, not the duration fat soluble vitamins, and some minerals, but has less of time at the breast, or the time interval between fat and sugar than mature breast milk (Riordan, 2010). breastfeedings. The breast milk secreted at the early It is produced in tiny volumes; in the first 3 days part of a breastfeeding, sometimes referred to as postpartum the volume per feeding ranges from 2 “foremilk”, has high water and carbohydrate content to 30 ml (Lawrence, 2010). The higher concentrations to help hydrate and energize the infant. It is also a of proteins and minerals gradually change over source of water-soluble vitamins (Riordan, 2010). The the first few weeks to reflect the infant’s needs as breast milk produced towards the latter part of a breastfeeding becomes well established and more breastfeeding, sometimes referred to as “hindmilk”, mature breast milk is produced (Riordan, 2010). has progressively higher fat content, although the Changes continue in later lactation when increases in fat content of both “fore” and “hind” breast milk sodium and protein, including lactoferrin, lysozyme increases as the breast is emptied (Daly et al., 1993). and secretory IgA (Dewey, 1984; Goldman, 1984) increase There is no clear distinction between foremilk and to enhance protection from infection for the older hindmilk. The composition also varies with the time infant and . of day and between breasts (Lawrence, 2010).

References

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Daly, S.E., Owens, R.A., Hartmann, P.E. (1993). The short-term synthesis and infant-regulated removal of milk in lactating women. Experimental Physiology, 78(2), 209–220.

Daly, S.E., Di Rosso, A., Owens, R.A., Hartmann, P.E. (1993). Degree of breast emptying explains changes in the fat content, but not fatty acid composition, of human milk. Experimental Physiology, 78, 741–755.

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